Manufacture of semiconductor ribbon and solar cells

ABSTRACT

A method is provided for producing solar cells employing slightly curved or nearly flat monocrystalline silicon ribbons. The ribbons are formed by cutting or slicing monocrystalline hollow tubes along their lengths, the tubes having been formed according to crystal growing processes disclosed in U.S. Pat. No. 3,591,348.

The present invention relates to the art of converting solar energy intoelectrical energy and more particularly to improved processes forforming substantially monocrystalline silicon for use in solar cells,and for forming solar cells, and to the resulting solar cells.

It is well known that radiation of an appropriate wavelength falling ona P-N junction of a semiconductor body serves as a source of externalenergy to generate hole-electron pairs in that body. Because of thepotential difference which exists at a P-N junction, holes and electronsmove across the junction in opposite directions and thereby give rise toflow of an electric current that is capable of delivering power to anexternal circuit. Most solar cells are made of silicon but cells made ofother materials, e.g. cadmium sulfide and gallium arsenide, have alsobeen developed and tested. Silicon is a favored material since it has aband gap of approximately 1.1 electron volts and thus responds quitefavorably to electromagnetic energy having a wave-length in the visibleand ultraviolet regions of the spectrum.

At the state of the art prior to this invention, solar cells are mostcommonly fabricated using semiconductor-grade silicon monocrystals (orother suitable semiconductor materials as known in the art) insubstantially flat ribbon form. The silicon ribbons may be providedinitially by growing the latter from a melt according to the processdisclosed in U.S. Pat. No. 3,591,348 issued to Harold E. LaBelle, Jr.Using the process disclosed by LaBelle, monocrystalline silicon bodiesmay be grown having controlled and predetermined cross-sectional shapes,e.g. round rods and tubes and flat ribbons, by means of so-calledcapillary die members which employ capillary action for replenishing themelt consumed by crystal growth. In the process described in U.S. Pat.No. 3,591,348 the crystal is pulled from a thin film of melt which issupported on the upper end surface of a die member that has one or morecapillaries for feeding melt to its upper end surface from a reservoirpool so as to automatically replenish the film. The film fully coversthe end surface of the die member and, since crystal growth occurs fromthe full expanse of the film, the growing crystal has a cross-sectionalshape substantially corresponding to the end configuration of the upperend surface of the die member. The process disclosed in U.S. Pat. No.3,591,348 is frequently described as the "EFG" process where the term"EFG" is an abbreviation for "Edge-defined, film-fed growth".

Silicon ribbons employed in solar cells must be substantiallymonocrystalline, uniform in size and shape and substantially free ofcrystal defects. It is not difficult to control the size and shape ofsubstantially monocrystalline ribbons grown by the EFG process. However,one problem which results during the production of flat elongatemonocrystals grown from the melt is the formation of defects adjacentthe ribbon edges. Although not known for certain, it is believed thatsuch edge defects result from the shape of the liquid/solid interface atthe ribbon edges or the accumulation adjacent the crystal edges ofimpurities present in the melt. These edge defects are objectionable andthe ribbons must be processed further to remove the defects before theycan be used.

Accordingly, the primary object of this invention is to provide arelatively simple and inexpensive method for producing semiconductorgrade silicon ribbon (or other suitable semiconductor material) for usein fabricating solar cells or other semiconductor devices.

Another object is to provide relatively inexpensive, high qualitysilicon of the character described.

Still other objects are to provide a new and improved method forproducing high quality solar cells.

The foregoing and other objects are achieved by a manufacturing methodwhich basically comprises first producing a substantiallymonocrystalline tubular body of silicon or other suitable semiconductormaterial and then cutting the tubular body along its length to produce aplurality of nearly flat, monocrystalline ribbons. Preferably thecutting of the tubular body is achieved by etching, as with an acid jet.In a preferred mode of practicing the invention to produce solar cells,the tubular body is treated to form an annular rectifying junction, thenit is severed longitudinally to form a plurality of nearly flat ribbons,and finally the ribbons are modified to form solar cells.

Other features and many of the attendant advantages of this inventionare set forth in or rendered obvious by the following detaileddescription which is to be considered together with the accompanyingdrawings wherein:

FIG. 1 is a perspective view with a portion broken away of a tubularmonocrystalline silicon body at a first stage of the manufacture of asolar cell in accordance with the present invention;

FIG. 2 is a perspective view with a portion broken away of themonocrystalline body of FIG. 1 at a second stage of solar cellmanufacture;

FIG. 3 is a perspective view with a portion broken away of themonocrystalline body of FIG. 1 at a third stage of solar cellmanufacture;

FIG. 3A is an enlarged end view of a portion of the body shown in FIG.3;

FIG. 4 is a perspective view with a portion broken away of themonocrystalline body of FIG. 1 at a fourth stage of solar cellmanufacture;

FIG. 5 is a perspective view with a portion broken away of a preferredform of solar cell constructed in accordance with this invention; and

FIG. 6 is a perspective view of a portion of a tubular monocrystallinebody showing an alternative method of forming individual nearly flatribbons.

In the drawings, like numerals refer to like parts.

As is well known to persons skilled in the art, substantiallymonocrystalline silicon bodies of selected cross-sectional shape can bereadily produced by the process described and claimed in U.S. Pat. No.3,591,348 issued to Harold E. LaBelle, Jr., using die members made ofgraphite and graphite dies coated with silicon carbide (see T. F.Ciszek, Edge-defined, Film-fed Growth of Silicon Ribbons, Mat. Res.Bull, Vol., 7, pps. 731-738, 1972). Substantially monocrystallinesilicon bodies of tubular shapes may be grown by means of dies shapedlike the dies shown in said U.S. Pat. No. 3,591,348 and also U.S. Pat.No. 3,687,633, issued Aug. 29, 1972 to Harold E. LaBelle, Jr., et al. Bycontrolling the growth environment and using a semiconductor grade melt,it is possible to grow tubular, substantially monocrystalline bodies ofsilicon with a purity suitable for semiconductor purposes. Also byintroducing suitable conductivity-type-determining impurities, i.e.dopants, to the melt it is possible to produce tubular bodies by theaforesaid processes which have a P- or N-type conductivity and apredetermined resistivity. The addition of a dopant to a melt from whicha crystal is grown in conventional, for example, with Czochralski-typeprocesses and also is exemplified by U.S. Pat. Nos. 3,129,061 and3,394,994.

Since a tubular body is continuous in cross-section, it has no edgeregions comparable to the long side edges of a ribbon. Accordingly,tubular bodies do not have the edge surface defects as normally found inflat ribbons or other shapes having two or more defined side edges. Moreprecisely, a tubular body grown by the EFG process has bettercrystallinity than ribbons grown by same EFG process under the sameconditions. In addition, the absence of edges leads to better stabilityduring growth, permitting greater growth flexibility and hence qualityof the crystals. Furthermore tubes of silicon can be grown at quite highgrowth rates.

Accordingly the essence of this invention is to produce ribbon-likebodies for use in making semiconductor devices by first growing asubstantially monocrystalline tubular body and then slicing the tubelengthwise into a plurality of ribbon-like pieces which may be used toform the devices.

While the invention may be used to provide ribbon-like bodies of avariety of materials, the following description illustrates theproduction of solar cells using silicon as the semi-conductor material.

In the preferred mode of practicing this invention, a tubular body ofone type conductivity is provided initially, and such body is thentreated to provide a zone of opposite type conductivity with arectifying junction created between such zones. The zone of oppositetype conductivity may be formed in various ways known to persons skilledin the art, e.g. by diffusion or ion implantation of dopants or byepitaxial deposition of opposite type conductivity material. Preferably,the zone is formed by diffusing a suitable dopant into the body. Thus,if the hollow body is a P-type semiconductor, a suitable N-type dopantis diffused into it to create an N-type conductivity zone. Similarly, ifthe hollow body is an N-type semiconductor, a suitable P-type dopant isdiffused into it to create a P-type conductivity zone. The choice ofdopant used depends on the material of which the hollow body is composedand also its conductivity type. Thus, for example, boron may be diffusedinto N-type silicon to produce a zone of P-type conductivity whilephosphorus may be diffused into P-type silicon to produce a zone ofN-type conductivity. The several types of dopants used for modifying theconductivity of silicon and how such conductivity-modifying impuritiesmay be diffused into a silicon body are well known (see, for example,U.S. Pat. Nos. 3,162,507; 3,811,954; 3,089,070; 3,015,590; and3,546,542). The types of dopants required to modify the conductivitytype of other materials, e.g. gallium arsenide, cadmium telluride, etc.,also are well known to persons skilled in the art. In accordance withprior art knowledge, the concentration of dopants in the P- andN-regions of the tubular structures is controlled to obtain the desiredresistivity of the P- and N-type regions. For solar cells, theresistivity of such regions is held to less than about 100 ohm-cm andfor best conversion efficiency is between about 0.001 to about 10ohm-cm; also in order to improve the efficiency of collecting thephotoelectrically produced carriers, the depth of the P-N junction fromthe surface which is to serve as the radiation receiving surface, ismade small, preferably in the order of 1/2 micron.

After the P-N junction is formed, the hollow body is sliced or cutlengthwise to produce a plurality of slightly curved or nearly flatelongated silicon bodies. The nearly flat elongated bodies are thenprovided with ohmic contacts or electrodes for their P- and N-typezones, whereby the resulting solar cell units may be connected to anexterior circuit.

If desired, the solar cells may be coated with a suitableanti-reflection or interference film to reduce reflection losses ofsolar radiation or to block absorption of infrared radiation.

An example of the preferred mode of practicing the invention will now bedescribed with reference to FIGS. 1-5. Turning first to FIG. 1, atubular body 10 of a substantially monocrystalline P-type silicon isprovided by growing it from a boron doped, semiconductor grade siliconmelt under an inert atmosphere using the above-described EFG process.The tubular body is grown from a melt contained in a quartz crucible(not shown) using a die (not shown) consisting of two graphite cylindersdisposed concentrically one inside the other and locked together in themanner of the two sleeves 24 and 26 of FIG. 1 of U.S. Pat. No.3,687,633. The gap between the two graphite cylinders is sized to serveas a capillary for molten silicon and the die assembly is disposed sothat melt can enter the bottom end of the capillary and rise to itsupper end by capillary action. This tubular body 10 is then introducedinto a diffusion furnace where it is exposed to a gaseous mixture ofoxygen and phosphorous oxychloride at a temperature of about 1000° C fora period of about 15 to 30 minutes. As a consequence of this diffusionstep, phosphorous is diffused into the outer and inner surfaces of thetube so as to form an N-P-N structure (see FIGS. 2 and 3A) withrelatively shallow outer and inner N regions 12 and 14 and thin layers16 and 18 of silicon dioxide covering the outer and inner surfaces. TheN regions 12 and 14 each have a depth of about 0.5 microns and thediffusion oxide layers each have a thickness of about 3000A. Theformation of the diffusion oxide layers results from the presence ofoxygen which is used as the transport medium for the phosphorousoxychloride.

Thereafter as shown in FIG. 2, the outer and inner surfaces of the tubeare coated with a conventional polymethylmethacrylate positive resistmaterial as represented at 20 and 22 (for convenience of illustrationthe N regions 13 and 14 and the oxide layers 16 and 18 are notspecifically shown in FIGS. 3 and 4). Then the outer photoresist layer20 is exposed to a narrow light beam, so that a plurality ofcircumferentially spaced, straight and narrow longitudinally extendingareas of the resist coating 20 are exposed to the beam and therebyaltered to a lower molecular weight polymer. The tube is then immersedin a preferential solvent or etchant such as methyl isobutyl ketone,with the result that the unexposed portions of the resist coating 20remain intact while the exposed areas are dissolved away as representedat 24 in FIG. 3 to expose narrow line portions of the outer oxide layer16.

The next step involves etching the tube so as to subdivide it into aplurality of narrow strips 26 as shown in FIG. 4. This is achieved intwo stages. In the first stage the tube is immersed in HF at roomtemperature for about 1-2 minutes so as to dissolve the exposed narrowportions of the outer oxide layer 16. In the second stage the tube isimmersed in KOH at room temperature for about 10 minutes (this timebeing determined by the tube thickness (or in a mixture of one part HFand three parts HNO₃), whereby the silicon tube is etched into precisewidth ribbon-like sections 26. Depending upon the tensile strength ofthe inner resist layer 22 and its adherence to the tube, the sections 26may or may not detach themselves from that layer when the etchant hasdissolved through the full wall thickness of the tube. In any event,etch cut sections 26 are removed from the etchant bath andtrichloroethylene is applied so as to dissolve away the inner resistlayer from each section. Then the ribbon-like sections 26 are immersedin HF followed by KOH (or a mixture of HNO₃ & HF) at room temperaturefor a period of about 2-3 minutes. This etch step serves to remove theirinner oxide layers and their inner N conductivity regions 14.

Thereafter trichloroethylene is applied to each ribbon-like section 26to dissolve away its outer resist layer 20 and then the sections 26 areagain immersed in HF at room temperature long enough (about 2-3 minutes)to remove the outer oxide layer 16 but not the outer N-conductivityregion 12.

The final step is to apply electrodes to the outer and inner surfaces ofthe sections 26 (FIG. 5). The electrodes are formed by a conventionalmetalization technique. Preferably the electrodes are nickel and areapplied by electroless plating. Alternatively the electrodes may belaminates formed by evaporation deposition and comprise a layer ofaluminum attached to the silicon body and a layer of silver bonded tothe aluminum layer. Other electrode materials also may be used and theelectrodes may be formed by other techniques known to persons skilled inthe art. As shown in FIG. 5, the electrode 30 on the outer surface ofthe ribbon-like body 26 is formed as a grid with relatively wide sideand end sections 32 and relatively narrow transverse sections 34 spacedso that a major portion of the outer surface 38 of the silicon body isuncovered and thus exposed to receive solar radiation. The otherelectrode 36 preferably covers the entire expanse of the inner surfaceof the silicon body. The resulting structure is a solar cellcharacterized by a substantially planar P-N junction represented by thedotted line 40 that lies close to the outer, i.e. upper, surface of thecell and electrodes 30 and 36 for coupling the cell into an electricalcircuit.

The presence of the silicon dioxide layers 16 and 18 is advantageous inthat the layers help to protect the silicon tube 10 in the event of anybreakdown of the photoresist layers 20 and 22 to the etchant. On theother hand, the silicon dioxide layers are not required if thephotoresist is applied with sufficient care to protect the inner andouter surfaces of the tubular body from being attacked by the etchantexcept along the areas 24 as above described. The formation of the oxidelayers can be avoided by using nitrogen instead of oxygen as thetransport medium for the phosphorous dopant. The formation of the oxidelayers also can be avoided by diffusing phosphorous into the tubularbody by means of phosphene gas which can be introduced into a diffusionfurnace without having to be admixed with any transport medium.

The formation of silicon dioxide on the layers at the inner and outersurfaces of the body also can be achieved when the P-N junction isformed by ion-implantation rather than by diffusion. Theion-implantation is carried out in a vacuum so that no oxides areformed. After the ion-implantation has been completed, the tube isannealed in an oxygen furnace at a temperature of about 1000° C for aperiod of about 15 minutes whereby silicon dioxide layers are formed atthe inner and outer surfaces of the body. The annealing process isconducted so as to maintain the diffusion oxide layers to a thickness ofabout 2 to 5 microns. The ion-implantation approach offers the advantagethat the dopant is introduced only at the outer surface of the tubularbody, thereby omitting the need for removing an inner oppositeconductivity region corresponding to the N-type region 14. With theion-implantation approach, the tube 10 is converted so that, beginningat its inner side and terminating at its outer side, it comprises aninner oxide layer, a P-type layer, a junction, an N-type layer, and anouter oxide layer. The etch cutting of a tube which has been subjectedto ion-implantation is essentially the same as the etch cuttingtechnique required for tubes which have been subjected to diffusiondoping. Specifically, the resist material is applied as coatings 20 and22 to the inner and outer surfaces of the tube, the areas 24 are formedby exposing and dissolving the resist as previously described, and theelongate nearly flat segments 26 are formed by submerging the tubularbody in a suitable etchant. The inner and outer electrodes are formed onthe sections 26 after the resist coatings and the inner and outerdiffusion oxides have been removed as previously described.

It is to be understood of course, that even if the tube is subjected toion-implantation, the formation of silicon dioxide layers at the innerand outer surfaces of the tubular body may be avoided by annealing thetube in a nitrogen rather than an oxygen atmosphere.

It is understood of course that the tubular bodies could be slicedlengthwise into ribbon-like sections as shown at 26 by subjecting thetubular body to the action of a mechanical cutting means rather than anetchant.

A further alternative method for cutting the tubular body into nearlyflat ribbons is illustrated in FIG. 6. In this case, a tubular body 10Aof one type conductivity material, e.g. N or P-type silicon, isprocessed so as to form a P-N junction 40A near its outer surface. Thetubular body is then cut by impinging onto its outer surface a fine jet46 of a selected etch solution, e.g. HF and HNO₃ or KOH in the case ofsilicon. The jet of etchant is directed onto the tubular body via anozzle 48 which is connected to a supply of etchant. The nozzle and thetubular body are moved relative to one another so that the jet ofetchant traverses the tubular body lengthwise and thereby causes thetube to be sliced longitudinally. The tubular body 10A is indexed aboutits longitudinal axis so that the jet 46 can slice the tubular body atselected circumferentially spaced regions. The jet etch cuttingtechnique is not described in greater detail since the technique is wellknown in the art and is described, for example, by C. R. Booker and R.Stickler, British Journal Applied Physics, 1962, Volume 13, page 446. Ifsilicon oxides exist on the outer and/or inner surfaces of the tube,they may be removed by means of a suitable etchant as previouslydescribed before or after the tube is cut by the jet cutting techniquedescribed above.

One skilled in the art will appreciate that if the tubular bodies 10 areinitially grown to a suitable diameter, they may be slicedlongitudinally so as to form elongate bodies which have the generalappearance of ribbons but that are characterized by a circular butgentle cross-sectional curvature instead of being flat. By way ofexample, a 2 inch diameter silicon tube may be cut into six 1 inch widesections which have a rise of about 1/8 inch. The curvature incross-section may be sufficiently gentle for the resulting solar cellsto be used as replacements for solar cells made from flat ribbons.Moreover it is contemplated that for certain applications a solar cellwhich has a gentle circular curvature to its radiation receiving surfacemay be more advantageous than a conventional flat solar cell.

The most significant advantage of the invention is that the resultingribbon-like sections 26 are substantially free of so-called "edgedefects". Eliminating such edge defects enhances the overall efficiencyof the resulting solar cell. A further advantage results from the factthat tubular bodies may be grown by the EFG technique at a pulling ratesubstantially the same as the pulling rate for flat ribbons, with theresult that the productivity of ribbon for manufacture of solar cells isincreased if tubular bodies are grown in place of ribbons and thensevered as herein described. The growth of tubular shaped bodies by theEFG process is easier than the growth of two dimensional ribbon shapedmembers.

It is to be noted that the foregoing description illustrates theproduction of solar cells commencing with the formation or provision ofa substantially monocrystalline tube having a substantially circularcross-section. However, one skilled in the art of growing crystals bythe EFG technique will appreciate that the advantages of the presentinvention can also be realized by starting with a tube of oval orpolygonal geometry, and slicing such tubes to form flat or nearly flatribbons.

Another possible modification comprises the following steps: (a)providing tubular bodies as above-described, (b) slicing the bodieslongitudinally into ribbon-like sections, and (c) then processing theindividual sections to form P-N junctions in accordance with knowntechniques, followed by formation of front and back electrodes asabove-described.

Obviously the invention may be practiced by using N-type silicon tubesand introducing a P-type layer or zone to the tube so as to form therequired P-N junction. Also the tubes and solar cells may be made ofsome other material beside silicon, e.g. cadmium telluride. Obviously nodoping of the tubes or ribbons is required in the case of sapphireribbons to be used as substrates for silicon integrated circuit devices.

Still other modifications and advantages will be obvious to one skilledin the art. Thus, for example, it is contemplated that the electrodescould be formed on the silicon tubes prior to cutting rather thanforming them on the separated sections 26. Of course, any diffusionoxides present on the tube would have to be removed before theelectrodes could be deposited on the tube. A further modificationconsists of substituting an etch-resistant wax for the photo-resistcoating and removing selected portions of the wax by scribing them awaywith a suitable tool so that narrow like areas of the tube are exposedas at 24 in FIG. 3. After the tube has been cut into sections 26 byexposing the areas 24 to the etchant as previously described, the wax isremoved from the sections by means of a suitable organic solvent, e.g.nephtha, toluene, etc. Still other modifications will be obvious topersons skilled in the art.

It is to be understood that the term "substantially monocrystalline" asused herein is intended to embrace a crystalline body that is comprisedof a single crystal or two or more crystals, e.g. a bicrystal ortricrystal, growing together longitudinally but separated by arelatively small angle (i.e. less than about 4°) grain boundary.

What is claimed is:
 1. A method of producing ribbon-like substantiallymonocrystalline bodies for use in fabricating solar cells comprising thesteps of:(a) providing a tubular substantially monocrystalline body of asemiconductor material; (b) forming a photovoltaic junction in thetubular body; and then (c) dividing said tubular body lengthwise into aplurality of ribbon-like bodies.
 2. Method according to claim 1 whereinsaid tubular body has a substantially circular cross-section.
 3. Methodaccording to claim 1 wherein said photovoltaic junction is formed bydiffusing a dopant into the tubular body.
 4. Method according to claim 1wherein the photovoltaic junction is formed in the tubular body byion-implantation.
 5. Method according to claim 1 wherein saidmonocrystalline hollow body is divided by dissolving selected portionsthereof with a liquid solvent.
 6. Method according to claim 5 includingthe step of coating selected areas of the tubular body with a materialthat is resistant to said solvent, so that said predetermined areas areprotected against dissolution by the solvent.
 7. Method according toclaim 1 including the steps of forming a protective film on the surfacesof the tubular hollow body, and removing selected portions of saidprotective film, and dividing said tubular body into a plurality ofribbon-like bodies by etching said tubular body where the selectedportions of the protective film were removed.
 8. Process according toclaim 7 wherein said protective film comprises a wax, and said linedportions are removed by scribing.
 9. Method of forming solar cellscomprising the steps of:(a) producing a tubular substantiallymonocrystalline body of a selected semiconductor material, (b) forming aphotovoltaic junction near the outer surface of said tubular body; (c)dividing said tubular body longitudinally into a plurality of elongatebodies each having first and second surfaces which constitute parts ofthe outer and inner surfaces respectively of said tubular body, and aphotovoltaic junction near said first surface; and (d) formingelectrodes on said first and second surfaces.
 10. Method of claim 9wherein said tubular body has an oxide layer at its outer and innersurfaces after said junction is formed, and further including the stepof removing each oxide layer before formation of the electrodes. 11.Method of claim 10 wherein said tubular body is divided by etching awayselected longitudinally extending areas thereof.
 12. Method of claim 11wherein the division of the tubular body comprises the steps of coatingthe tubular body with a photoresist, removing the photoresist from saidselected areas, and contacting said tubular body with an etchant whichis capable of dissolving said tubular body and is inert with respect tosaid photoresist.